Electrooptic materials, such as lithium niobate, are extremely useful in the telecommunications industry for modulating and demodulating signals carried by an optical carrier beam. For example, to modulate an optical beam, an electrode or a multi-electrode structure, such as a coplanar waveguide (CPW) electrode structure, exposes an optical waveguide disposed with an electrooptic substrate to a time-varying (typically RF) electric field. The RF field varies the index of refraction of the electrooptic material of the optical waveguide, changing the phase of the beam propagating along the waveguide, thus modulating the beam. As is known in the art, it is often advantageous to arrange such a modulator as an interferometer wherein the CPW electrode structure applies the RF electric field to two optical waveguide lengths in a "push-pull" fashion. Beams propagating along the two optical waveguide lengths are combined to interfere to produce a single optical output. Techniques known in the art for forming optical waveguides with electrooptic substrates include titanium indiffusion and an annealed proton exchange (APE.TM.) technique.
Devices such as modulators and detectors are typically operated at a selected bias point. As is understood by those of ordinary skill in the art, depending on the circumstances and device configurations, a bias point can be selected such that the device operates within a particular linear range, at a minimal zero throughput, or at a half power point of optical output.
According to the prior art, electrooptic devices are typically biased by attempting to apply a known constant electric field to the optical waveguide(s) formed in or on the electrooptic substrate, such as by applying a fixed d.c. bias voltage to an appropriately located bias electrode. In some instances, the bias voltage can be applied to the some or all of the electrodes that apply the RF fields. Unfortunately, voltage biasing techniques must deal with the pheonomenon of bias drift. Although a constant voltage is applied to the biasing electrode, the actual electric field applied to the electrooptic optical waveguide varies, and the bias point of the device drifts. Physical impurities, crystal defects, and any causes of both trapped and mobile charges are considered to affect the bias stability of the device. In addition, because the optical waveguides are typically located near the surface of the electrooptic substrate, the crystal composition near the surface affects drift of the bias point via a variety of surface chemistry mechanisms. Even the method used for fabricating the waveguides, often involving indiffusion or proton exchange processes, can affect bias point drift, because these techniques modify the crystal structure. Bias point drift is a known problem and extensively discussed in the technical literature, particularly regarding lithium niobate, the most common electrooptic material used for optical devices.
In one approach to countering bias drift in interferometer-type modulators, the d.c. voltage is not fixed. A feedback circuit monitors the bias point, i.e. the phase or intensity of the output beam, and adjusts the bias voltage applied to the modulator. However, the voltage available for bias is typically limited, for example, to the "rail" voltage of 15 volts. It is possible that a feedback circuit, in tracking and correcting for drift, could "hit the rail," that is, apply the full 15 volts, and to correct the drift will decrease the voltage by some step, essentially going to the next "fringe" of the interferometer. Such a "reset" is considered undesirable as it can result in lost data. Reset can usually be avoided by proper design, but it remains a concern, and compensating for the possibility of reset can complicate the biasing circuit design.
In some instances the optical devices can be manufactured to operate at a selected bias point. For example, the two optical waveguide lengths of an interferometer device can be fabricated having different physical lengths to introduce a selected phase difference between beams propagating along the lengths. This technique is effective and can increase cost, but may be limited to use in a specific application.
Accordingly, it is an object of the present invention to overcome one or more or the aforementioned drawbacks and disadvantages of the prior art.
This and other objects of the invention will in part appear hereinafter and in part be apparent to one of ordinary skill in light of the disclosure herein.